$OpenBSD: patch-include_SAX_helpers_AttributesImpl_hpp.orig,v 1.1 2017/05/21 09:50:45 espie Exp $
Nope, you can't add numbers to strings.

Index: include/SAX/helpers/AttributesImpl.hpp
--- include/SAX/helpers/AttributesImpl.hpp.orig
+++ include/SAX/helpers/AttributesImpl.hpp
@@ -547,7 +547,7 @@ class AttributesImpl : public Attributes<string_type, 
   {
     // sort out
     std::string msg =
-	    "Attempt to modify attribute at illegal index: " + index;
+	    "Attempt to modify attribute at illegal index"; // + index ????
     throw std::out_of_range(msg);
   }
 
